Electrophysiological and optical recording techniques are used primarily to elucidate the development, differentiation and cellular distribution of physiologically important properties expressed in vitro by vertebrate CNS neurons. Electrical studies involve direct, high-fidelity amplification of ion fluxes generated either in single cells or patches or in synaptically coupled pairs of cells maintained in monolayer culture. Optical recordings include indirect measurements of membrane potential or of intracellular ion concentration in small populations (50-100) of cultured cells. Principal findings this year include: 1) in the embryonic rat spinal cord voltage-dependent Na+ channels critical for propagation of activity patterns in networks emerge before depolarizing GABA-A receptor-activated Cl- channels, which appear before depolarizing glutamate (kainate-type) receptor-activated cation channels-, 2) these depolarizing conductances are differentiated sequentially on the majority of all spinal neurons; 3) activation of depolarizing conductances stimulates an increase in free intracellular Ca2+ via opening of depolarization-activated Ca2+ channels; 4) the biophysical properties of GABA-activated Cl- channels recorded in spinal cord neurons differ from those observed at the same embryonic day in olfactory neurons; 5) Cl- ion channels open spontaneously and randomly in spinal and hippocampal neurons, thus contributing to its baseline properties and potential; 6) pacemaker activity driven by GABA occurs in spinal neurons; 7) Cl- channel activity can be eliminated in hippocampal neurons by a gentle stream of bathing saline applied to the cell surface and by application of antagonists at GABA-A receptor-coupled Cl-channels; 8) after antagonism of Cl- channel activation, the stream of saline becomes ineffective, indicating that a diffusible substance, probably GABA, acts at the cell surface to stimulate the spontaneous Cl- ion channel activities; 9) application of Zn in the bathing saline depresses spontaneous Cl- ion channel activity in both spinal and hippocampal neurons; 10) in hippocampal neurons, Zn exposure leads to transient GABAergic signals whose kinetics of decay are identical to those describing the life-times of Cl- channels; 11) Zn transforms GABA release from random to synchronized; 12) all-or-none "miniature" GABAergic transients reflecting the release of an elementary, uniform amount of GABA are summed to generate the properties of the post-synaptic signals; 13) low-frequency release rates generate low-intensity postsynaptic signals with highly variable properties, while high-frequency release creates high-intensity signals with more precise properties.